Elementary Reactions
Elementary reactions occur in a single encounter
Unimolecular: A Rate = k[A] Bimolecular: A + B Rate = k[A][B] Termolecular: A + B + C Rate = k[A][B][C]
Termolecular reactions are rare; higher molecularities are unknown.
For elementary reactions, reaction order equals molecularity ConcepTest 1
Which of the following reactions is not an elementary reaction?
A. H2S + O2 H2O + SO B. CH4 + F HF + CH3
C. NO + NO3 2NO2 + + D. He + N2 N + N + He ConcepTest 1
Which of the following reactions is not an elementary reaction?
A. H2S + O2 H2O + SO B. CH4 + F HF + CH3
C. NO + NO3 2NO2 + + D. He + N2 N + N + He Composite Reactions
Composite reactions involve two or more elementary steps
Composite reactions are likely when:
1. Complex rearrangements occur
2. More than two molecules of reactants are involved
3. The rate equation does not correspond to the stoichiometric equation
4. Reaction intermediates are detected ConcepTest 2
Is the rate of an overall composite reaction lower, higher, or equal to the average rate of the individual steps in the mechanism?
A. Lower B. Higher C. Equal ConcepTest 2
Is the rate of an overall composite reaction lower, higher, or equal to the average rate of the individual steps in the mechanism?
A. Lower B. Higher C. Equal Temperature Dependence of k
Many reactions have a rate cons’t that shows a temperature dependence as on the left.
This corresponds to the form kAe E*/RT where A and c are simply empirical constants. E* has units of energy, and k has units of the rate constant. Might A and E* have physical significance? Arrhenius Equation temperature dependence of k
kAe E/RTa A = pre-exponential factor
Ea= activation energy Potential energy difference between products and reactants should be related to Hreaction Let’s try to develop a 2nd order rate constant along these lines A + B k C + D an elementary reaction
k should be proportional to number of times A and B collide per second multiplied by fraction with sufficient energy for rxn multiplied by fraction with that energy contained in the appropriate degrees of freedom for rxn multiplied by fraction of collisions that occur with a geometry appropriate for rxn There might or might not be a barrier. How do we think about this? The Arrhenius equation Insight from collision theory
kAe E/RTa
A = frequency factor (collisions with proper orientation)
e E/RTa = f = fraction of collisions with sufficient energy to surmount barrier Pre-exponential Factor, A
A = PZAB
ZAB = collision density (calculated earlier)
P = steric factor (the probability that colliding molecules have the proper orientation) Values of P vary from 1 for atoms to 10–6 for biomolecules Steric Factor, P molecular orientation
NO + NO3
NO2 + NO2
More in a few minutes E a The Exponential Factor, e RT A + B C + D an elementary reaction
Consider possible activation energies (reaction profiles) and steric effects for the following reaction:
CH4 + D CH3D +H
Steric hindrance: not much
Activation energy: yes How much? C-H bond 140 kJ/mol (but maybe a lot less!) A + B C + D an elementary reaction
Consider possible activation energies (reaction profiles) and steric effects for the following reaction:
OH + D HOD
Steric hindrance: some Activation energy: no Back to integrated rate laws
Now that we know about elementary reactions, we can look at how the integrated rate laws might apply to elementary processes. First, we would write the three 2nd order rxns as A + A P The first two are 2nd order from this A + B P perspective, while the third is a three- body (3rd order) process, and much less A + B + B P common. Rate laws
d A 2 k A A + A P dt d A A + B P k AB dt A + B + B P d A 2 k AB 11dt Case 1: kt AA0
Case 2: if [B] >> [A], then [B] is essentially constant, and AA ekt[B] 0 Pseudo first order reaction Reaction Mechanism
A detailed sequence of steps for a reaction
Reasonable mechanism: 1. Elementary steps sum to the overall reaction
2. Elementary steps are physically reasonable
3. Mechanism is consistent with rate law and other experimental observations (generally found from rate limiting (slow) step(s) A mechanism can be supported but never proven NO2 + CO NO + CO2
2 Observed rate = k [NO2] Deduce a possible and reasonable mechanism
NO2 + NO2 NO3 + NO slow
NO3 + CO NO2 + CO2 fast
Overall, NO2 + CO NO + CO2
Is the rate law for this sequence consistent with observation? Yes Does this prove that this must be what is actually happening? No!